Relative Prey Size Research Articles

To hold or not to hold? The effects of prey type and size on the predatory strategy of a venomous snake

AbstractThe dangerous prey hypothesis predicts that when predators can discriminate between harmless and dangerous prey, they should alter their predatory behavior according to the risk. Venomous snakes, which rely on an envenomating strike to kill prey, often feed on potentially dangerous prey such as rodents, and have the choice between two alternative strategies following a bite: they either hold onto prey until it is incapacitated, or release it immediately, relying on scent trailing to find the bitten prey. In this study, we combined observational and experimental data collected in the field in South Africa to test the hypothesis that prey type and size affected the predatory strategy of a venomous snake, the puff adder (Bitis arietans;Viperidae). Relative prey size, but not prey type, affected the snake's strategy: puff adders typically struck and held prey of small size (e.g., toads, small adult mice and shrews), but often struck and released larger prey (e.g., large adult mice and rats), presumably because retaliatory bites from the latter ones are more dangerous. Hence, puff adders discriminated according to prey size and adjusted their predatory strategy consequently. Contrary to the common belief that viperid snakes typically strike and release prey, our findings agree with empirical evidence found in the literature, that is, puff adders generally alter their predatory tactics based on prey size. Although a few laboratory studies have examined the theme of the present paper in other venomous snakes, our research was conducted in a natural setting and thus provides a better understanding of the factors used by predators to choose between alternative hunting strategies.

Scaling of dentition and prey size in the California moray (Gymnothorax mordax)

Scaling patterns of tooth morphology can provide insights on prey capture strategy and dietary patterns as species grow through ontogeny. We report the scaling of dentition and diet and how it relates to body size in the California moray, Gymnothorax mordax. We sampled lengths, widths, and curvature for teeth lining five distinct regions of the oral jaws across 21 G. mordax individuals ranging from 383 to 1110mm total length. Absolute tooth length in relation to moray size shows positive allometry only for the outer maxillary teeth, while teeth lining the inner maxilla display positive allometry in tooth base width. All other regions exhibit isometric growth in both length and width relative to moray size. Similar to previous descriptions of other moray species, the longest teeth in the oral jaws are the median intermaxillary teeth. This series of three teeth are depressible and rooted in the center of the ethmovomer, the bone that forms the roof of the rostrum. We hypothesize that caudal mobility of the median intermaxillary teeth aids in prey transport by enabling the pharyngeal jaws to remove pierced prey without requiring full abduction of the oral jaws. The predominantly isometric tooth growth in G. mordax suggests that the oral teeth grow proportionately as individuals increase in size. Stomach contents from the field suggest that G. mordax is highly piscivorous. While a strong positively allometric relationship between vertical gape and head length supports the expectation that moray increase relative prey size over ontogeny, we found no relationship between prey standard length and moray size. This suggests that while larger individuals are capable of consuming larger prey, individual G. mordax are opportunistic predators that do not specialize on prey of a specific size over ontogeny.

Does nutritional status constrain adoption of more costly and less risky foraging behaviour in an Amazonian shelter-building spider?

ABSTRACTIndividual nutritional status may increase marginal foraging costs and potentially drive animal foraging strategies. Here, we investigated how it might affect foraging strategies of the shelter-building spider Hingstepeira folisecens (Hingston 1932) (Araneidae). This spider can catch prey using two strategies with different relative costs: ‘pulling’ (higher energy expenditure) and ‘pursuing’ (lower energy expenditure). We conducted experiments by offering prey at a fixed distance from the shelter entrance to investigate if spiders with lower nutritional status would exhibit riskier but less costly foraging behaviours more frequently than spiders with better status. The nutritional status did not affect the foraging behaviours adopted by H. folisecens. Additionally, the frequency of both foraging behaviours was similar. Therefore, our results did not provide any evidence that individual nutritional status is a factor driving the selection of foraging behaviours in H. folisecens. We suggest that the malnourished spiders should adopt additional strategies to compensate for the costs of pulling behaviour, especially when prey is far from the shelter. Furthermore, it is possible that other factors, like relative prey size in relation to web size, can help us to understand the threshold that determines when such individuals must adopt one of two possible behaviours.

Natural History of Neonatal Green Anacondas (Eunectes murinus): A Chip Off the Old Block

Our knowledge of the biology of neonatal snakes has lagged behind that of adult animals, mostly due to the difficulty of finding and studying neonatal snakes in the wild. Traditional approaches view neonatal reptiles as miniature replicates of their adult counterparts. In this contribution, we present data on the natural history of neonatal Green Anacondas from opportunistic captures in the wild over a 17-year period, as well as from a brief study on captive-born radio-tagged individuals. Both approaches converge in presenting a picture of the ecology of neonatal anacondas showing many similarities between their natural history and that of adult anacondas in spite of the great size difference. The neonates' biology resembles that of adults, especially males, in their preference for birds in their diet, the relative prey size they choose, slow growth rates they experience, low feeding frequency, little mobility, and preference for similar habitats of stagnant, shallow water covered by aquatic vegetation. T...

Food habits of Arctic staghorn sculpin (Gymnocanthus tricuspis) and shorthorn sculpin (Myoxocephalus scorpius) in the northeastern Chukchi and western Beaufort Seas

Arctic staghorn sculpin (Gymnocanthus tricuspis) and shorthorn sculpin (Myoxocephalus scorpius) belong to Cottidae, the second most abundant fish family in the western Arctic. Although considered important in food webs, little is known about their food habits throughout this region. To address this knowledge gap, we examined and compared the diets of 515 Arctic staghorn sculpin and 422 shorthorn sculpin using stomachs collected over three summers in the northeastern Chukchi Sea (2010–2012) and one summer in the western Beaufort Sea (2011). We used permutational multivariate analysis of variance (PERMANOVA) and non-metric multidimensional scaling (nMDS) to compare sculpin diets between regions and selected size classes. Differences in mouth morphologies and predator size versus prey size relationships were examined using regression techniques. Arctic staghorn sculpin and shorthorn sculpin diet compositions differed greatly throughout the Chukchi and Beaufort Seas. Regardless of body size, the smaller-mouthed Arctic staghorn sculpin consumed mostly benthic amphipods and polychaetes, whereas the larger-mouthed shorthorn sculpin shifted from a diet composed of benthic and pelagic macroinvertebrates as smaller individuals to shrimps and fish prey as larger individuals. Within shared habitats, the sculpins appear to partition prey, either by taxa or size, in a manner that suggests no substantial overlap occurs between species. This study increases knowledge of sculpin feeding ecology in the western Arctic and offers regional, quantitative diet information that could support current and future food web modeling efforts.

Predator–prey mass ratio revisited: does preference of relative prey body size depend on individual predator size?

Summary Quantifying predator–prey body size relationships is key to understanding food webs. Food web models often assume that all individuals of predator species prefer the same relative body size of prey, using a single constant called preferred predator–prey mass ratio (preferred PPMR). In contrast, empirical studies have shown that relative prey body size in diet varies with individual predator size, challenging the food web models based on size‐invariant preferred PPMR and their predictions. We point out that this apparent inconsistency arises because empirical PPMR in those previous studies has been measured only through dietary data (i.e. realized PPMR rather than preferred PPMR) without considering the effects of environmental prey availability, suggesting the possibility that preferred PPMR may be in fact independent of individual predator size. Here, we present a new approach to revisit the assumption of size‐invariant preferred PPMR in food web models. The approach compares two measures of PPMR calculated from prey compositions in predator diet and environmental prey composition, respectively (i.e. realized PPMR vs. environmental PPMR). The deviations between realized and environmental PPMRs are considered as a proxy of individual variations in relative prey size preference (i.e. preferred PPMR). We apply this idea to long‐term dietary data of an omnivorous predatory fish species collected from a lake ecosystem over four decades. Our results showed that the preferred PPMR is independent of individual predator size when the foraging mode (i.e. the major prey type) of the predator is considered while the realized PPMR is size‐dependent regardless of the foraging mode, especially when analysed analogously to previous empirical studies. We suggest that the apparent inconsistency between theoretical assumption and empirical observation of PPMR is due to the conceptual and methodological confusion and could be resolved by distinguishing between preferred and realized PPMRs. Further, in contrast to the previous arguments based on realized PPMR, we provide the first empirical support for size‐invariant preferred PPMR. Future studies are encouraged to apply our ideas to other species/systems to test the robustness of size‐invariant preferred PPMR and to better describe food web models. A lay summary is available for this article.

Size-based approaches to aquatic ecosystems and fisheries science: a symposium in honour of Rob Peters

This special issue honours Rob Peters’ outstanding contributions to the field of aquatic ecology. It focuses on the size spectrum approach — in which individual organisms, rather than species, are the most basic biological unit — and highlights applications of this approach to fisheries management. The 21 papers in this issue cover three subject areas: (i) the use of size spectra to characterize variation in community structure, (ii) the development of size-based models of ecosystem dynamics to address fisheries questions, and (iii) applications of size-based theory to examine the consequences of variation in predator–prey size relationships, body size – trophic level relationships, and body size – life history relationships. The empirical studies herein demonstrate the utility of size spectra as indicators of population or community structure and for detecting impacts associated with environmental change. Future research focused on refining size-based sampling methods, standardizing metrics and analytical methods, understanding model sensitivity to the underlying assumptions, and comparative studies across ecosystems will enhance our ability to reliably interpret changes in size spectrum characteristics, thus facilitating their use as indicators of ecological change.

A length-based approach to predator–prey relationships in marine predators

Body size is a critical feature of the ecology of most organisms and has been used to describe and understand predator–prey interactions in both terrestrial and aquatic environments. Most previous studies have used prey mass to examine the relationships between predator size and prey size; however, using prey lengths may provide a different perspective, particularly for gape-limited fishes. Using a large database of predator and prey lengths for marine aquatic predators, I found the expected positive wedge-shaped relationship between predator length and prey length and a negative converging relationship between relative prey length (prey–predator length ratio = a measure of trophic niche breadth) and predator length. Distinct patterns in the size scaling of this measure of trophic niche breadth were identified using quantile regression: converging relationships were common among adults but absent among larvae. This difference suggests contrasting ontogenetic foraging opportunities between adults and larvae: a lack of large relative prey sizes for the largest adult predators, and a greater ability of larvae to include larger prey items in their diet as they grow.

Trophic niche overlap among dolphinfish and co-occurring tunas near the northern edge of their range in the western North Atlantic

We present the first quantitative analyses of dolphinfish (Coryphaena hippurus) foraging habits and trophic interactions with co-occurring yellowfin (Thunnus albacares) and albacore (T. alalunga) tunas in the Southern New England region of the western North Atlantic Ocean. Fish caught by recreational anglers in offshore waters of Massachusetts were sampled during the summers of 2007–2010. Diet analysis revealed that shortfin squid (Illex illecebrosus) and small pelagic crustaceans were principal prey to dolphinfish, yellowfin tuna, and albacore tuna. A wide variety of Sargassum-associated fishes were also important to dolphinfish and yellowfin tuna diets. Dietary (Schoener’s index: 0.82–0.86) and isotopic niche (isotopic ellipse overlap: 53.6–64.7 %) overlap was high, and dolphinfish and tunas occupied equivalent trophic positions (TP = 3.4–3.6). Relative prey size in dolphinfish and yellowfin tuna diets exhibited convergence with ontogeny. Overall, dolphinfish had the greatest isotopic niche width, which was twice as large as yellowfin tuna and three times as large as albacore tuna; dolphinfish also consumed the greatest range of prey sizes. Results quantify dolphinfish trophic interactions in the western Atlantic near the northern extent of their geographical range, and are relevant for ecosystem-based management of the offshore pelagic guild in the context of shifting fish populations and fisheries in response to climate and ecological change.

Energy and the Scaling of Animal Space Use

Daily animal movements are usually limited to a discrete home range area that scales allometrically with body size, suggesting that home-range size is shaped by metabolic rates and energy availability across species. However, there is little understanding of the relative importance of the various mechanisms proposed to influence home-range scaling (e.g., differences in realm productivity, thermoregulation, locomotion strategy, dimensionality, trophic guild, and prey size) and whether these extend beyond the commonly studied birds and mammals. We derive new home-range scaling relationships for fishes and reptiles and use a model-selection approach to evaluate the generality of home-range scaling mechanisms across 569 vertebrate species. We find no evidence that home-range allometry varies consistently between aquatic and terrestrial realms or thermoregulation strategies, but we find that locomotion strategy, foraging dimension, trophic guild, and prey size together explain 80% of the variation in home-range size across vertebrates when controlling for phylogeny and tracking method. Within carnivores, smaller relative prey size among gape-limited fishes contributes to shallower scaling relative to other predators. Our study reveals how simple morphological traits and prey-handling ability can profoundly influence individual space use, which underpins broader-scale patterns in the spatial ecology of vertebrates.

Ontogenetic Shifts in the Predatory Habits of the Northern Moonsnail (Lunatia heros) on the Northwestern Atlantic Coast

Naticid gastropods provide a highly tractable system for exploring ontogenetic changes in diet because the telltale boreholes they leave behind in their hard-shell molluscan prey can be linked reliably to naticid size. In this study, ontogenetic changes in diet and size selectivity across 7 common prey species of Lunatia heros (Say, 1822) were explored using a forensic approach based on beach-collected shells from the eastern coast of Nova Scotia, Canada. Laboratory experiments were also undertaken to assess the relationship between predator size and outer borehole diameter (OBD) for 3 of these prey species. The results confirmed a strong relationship between OBD and shell length for L. heros and demonstrated that this relationship was consistent across prey species varying in shell thickness and composition. Analyses of drilled, beach-collected shells revealed that L. heros clearly differentiates among prey with respect to size and species, with ontogenetic changes in prey selection appearing to reflect shifting predator—prey size relationships. Among prey species, larger predators demonstrated the capacity to select larger prey species and avoid smaller prey taxa. Similarly, within prey species, larger predators preyed on larger individuals of a given species, but also exploited a wider selection of prey sizes than smaller moonsnails. The moonsnail L. heros also exhibited borehole site specificity for both bivalve and gastropod prey, but did not discriminate between left versus right valves when drilling bivalve taxa. The results of this study are important for understanding the changing role of L. heros across its life span within soft-sediment communities. This research should now be broadened to encompass the full suite of prey species targeted by this drilling predator.

Slow swimming, fast strikes: effects of feeding behavior on scaling of anaerobic metabolism in epipelagic squid.

Many pelagic fishes engage prey at high speeds supported by high metabolic rates and anaerobic metabolic capacity. Epipelagic squids are reported to have among the highest metabolic rates in the oceans as a result of demanding foraging strategies and the use of jet propulsion, which is inherently inefficient. This study examined enzymatic proxies of anaerobic metabolism in two species of pelagic squid, Dosidicus gigas and Doryteuthis pealeii (Lesueur 1821), over a size range of six orders of magnitude. We hypothesized that activity of the anaerobically poised enzymes would be high and increase with size as in ecologically similar fishes. In contrast, we demonstrate that anaerobic metabolic capacity in these organisms scales negatively with body mass. We explored several cephalopod-specific traits, such as the use of tentacles to capture prey, body morphology and reduced relative prey size of adult squids, that may create a diminished reliance on anaerobically fueled burst activity during prey capture in large animals.

Consequences of Intraspecific Variation in Female Body Size inStagmomantis limbata(Mantodea: Mantidae): Feeding Ecology, Male Attraction, and Egg Production

Body size is an important feature of organisms, influencing many components of life history and fitness, such as feeding success and reproductive output. Body size is considered especially salient for solitary predators, whose food intake hinges on individual predation success, which in turn is often driven by the relative sizes of predator and prey. The current study examined intraspecific variation in adult female length and its fitness consequences in a solitary predator, the praying mantid Stagmomantis limbata Hahn. Through a 5-yr integration of observational and experimental work in the field and captivity, we investigated the relationship between female pronotum length and prey size, diet breadth, male attraction, and measures of egg production (fecundity and ootheca mass). We found that longer females ate longer prey in the field and showed greater breadth of prey size than shorter females. Longer females did not necessarily feed at higher rates in the field, as measured by the rate of abdominal expansion. Female length failed to show significant effects on male attraction or on the incidence of cannibalism. Longer females had higher fecundity (mature eggs in body at death) and laid heavier oothecae than shorter females. In nature, longer females consistently emerged as adults earlier in the season than shorter females. Shorter female adults emerged when feeding rates were higher in the field, suggesting an incidental ecological benefit of shorter adult size.

Macro-ecological patterns of a prey–predator system: rodents and snakes in West and Central Africa

Optimal foraging theory predicts that in animals both the sizes of prey consumed and the range of prey sizes eaten should increase with increasing predator size. Rodent prey and rodent-eating snakes (their predators) can provide a useful model for testing the ecological correlates of these predictions. In this study, we explore the relationships between species richness of these predators and prey and the prey size and predator size relationships. The analysis was performed at the level of the whole of West and Central Africa. Distribution data and body size for each genus within both rodents and snakes were obtained from the literature, and mapped using the World Map program. Numbers of rodent and snake genera by cell increased significantly from North to South, this being consistent with typical macro-ecological patterns. Snake mean body size per cell and mean rodent body size per cell were not significantly correlated; however, when the same analysis was restricted to only rodent-eating snakes, this re...

A new look at the Lake Superior biomass size spectrum

We synthesized data from multiple sampling programs and years to describe the Lake Superior pelagic biomass size structure. Data consisted of Coulter counts for phytoplankton, optical plankton counts for zooplankton, and acoustic surveys for pelagic prey fish. The size spectrum was stable across two time periods separated by 5 years. The primary scaling or overall slope of the normalized biomass size spectra for the combined years was −1.113, consistent with a previous estimate for Lake Superior (−1.10). Periodic dome structures within the overall biomass size structure were fit to polynomial regressions based on the observed sub-domes within the classical taxonomic positions (algae, zooplankton, and fish). This interpretation of periodic dome delineation was aligned more closely with predator–prey size relationships that exist within the zooplankton (herbivorous, predacious) and fish (planktivorous, piscivorous) taxonomic positions. Domes were spaced approximately every 3.78 log10 units along the axis and with a decreasing peak magnitude of −4.1 log10 units. The relative position of the algal and herbivorous zooplankton domes predicted well the subsequent biomass domes for larger predatory zooplankton and planktivorous prey fish.

Evolution of parental roles in provisioning birds: diet determines role asymmetry in raptors

Raptors are known for asymmetric parental roles, with the female brooding and feeding offspring, and the male providing food, usually assisted by the female from the latter part of the rearing period. Yet, the evolution of this role asymmetry is poorly understood. From filming prey delivery and handling in 61 nests of 9 raptor species, we show that the female role is related to prey size and prey type. Within species, nestlings were less likely to ingest larger prey unassisted, independent of increasing ability of unassisted feeding with increasing age. Feeding time increased with prey size and was longer for avian than for mammalian prey of the same size when nestlings fed unassisted. Across species, the female partitioned prey and fed offspring for a longer portion of the rearing period as prey size increased and as the diet contained more birds. Providing for the family selects for small body size; hence, extended female confinement as sedentary food processor for offspring would leave greater potential for differential selection on male and female body size. This may explain the female-biased size dimorphism among raptors, which becomes larger as diet changes from insects via reptiles and mammals to birds and as relative prey size increases. Symmetric parental roles and no female-biased size dimorphism would be expected in birds providing insects or other food that nestlings swallow without maternal assistance. Prey type and prey size would be important also for sexual conflicts and evolution of polygamy and mate desertion in birds providing for offspring.

Predator–prey body size relationships when predators can consume prey larger than themselves

As predator-prey interactions are inherently size-dependent, predator and prey body sizes are key to understanding their feeding relationships. To describe predator-prey size relationships (PPSRs) when predators can consume prey larger than themselves, we conducted field observations targeting three aquatic hemipteran bugs, and assessed their body masses and those of their prey for each hunting event. The data revealed that their PPSR varied with predator size and species identity, although the use of the averaged sizes masked these effects. Specifically, two predators had slightly decreased predator-prey mass ratios (PPMRs) during growth, whereas the other predator specialized on particular sizes of prey, thereby showing a clear positive size-PPMR relationship. We discussed how these patterns could be different from fish predators swallowing smaller prey whole.

Feeding in natricines: relationships among feeding morphology, behavior, performance and preferred prey type

AbstractThe diversity of feeding mechanisms among predators reflects phenotypic modifications that may improve feeding performance on a preferred prey type. I compared trophic morphology, feeding performance (time and upper‐jaw walks) and behavior (initial bite and ingestion directions) among three species of natricine snakes that were fed fish and frogs over a broad range of relative prey sizes. Feeding behavior was influenced by prey type but did not differ among the snake species. Both bite and ingestion directions influenced the number of upper‐jaw movements (lateral excursions and protractions) required to consume fish, but only initial bite position significantly affected the number of upper‐jaw movements required to ingest frogs. Within snake species, feeding performance did not differ between fish and frogs for Nerodia fasciata and Nerodia rhombifer; however, Thamnophis proximus consumed fish with fewer upper‐jaw movements. Feeding time differed significantly among snake species when fed both fish and frogs. Trophic morphology did not significantly affect ingestion costs for fish but did influence ingestion when fed frogs. In general, differences in trophic morphology among the three species are not correlated to handling and ingestion performance.

Effects of prey size structure and turbulence on feeding and growth of anchovy larvae

Foraging processes in plankton and plank- tivorous fish are constrained by relative prey and predator size and therefore, these are important varia- bles to include in a foraging model. The distribution of prey biomass across different size classes can be char- acterized by a size spectrum slope. We present a for- aging model for anchovy larvae including the most relevant processes such as prey encounter, capture- and pursuit success, all influenced by light, turbulence and prey characteristics. We modelled ingestion rates and specific growth rate by coupling the foraging model with an existing bioenergetic model, and per- formed a sensitivity analysis of prey ingestion in tur- bulent environments assuming either hemispherical or conical perceptive volume. Our results suggest that turbulence has no positive effect because of the low capture ability, small prey size and small visual vol- ume for anchovy larvae. The predicted ingestion is too low to sustain the growth potential of larvae when assuming conical perceptive volume even under prey densities substantially higher than normally found in the field. Ingestion rate is sensitive to the total biomass and the slope of the prey size spectra, specifically because it determines the abundance of prey around the optimal size for the larvae. The model also sug- gests that small larvae benefit from a prey size struc- ture with steep prey size-spectra slope while a large larva benefit from less steep slopes. The model can act as a link between size-spectra measurements from the field and the foraging conditions of larval anchovies.

The evolution of the stimulus control of constricting behaviour: inferences from North American gartersnakes (Thamnophis)

Behavioural traits that have evolved relatively recently can provide unique insights into the nature of early evolutionary stages and the adaptive significance of such traits. We used this “recent origins” approach by examining the stimulus control of constricting behaviour in Western Terrestrial Gartersnakes (Thamnophis elegans), a species in which constriction evolved within the last several million years. Using mice and frogs as prey, we tested the hypothesis that prey struggling is a key stimulus eliciting constriction. Mean prey struggling intensity was significantly higher for mice than for frogs, and the snakes often constricted mice but rarely constricted frogs. Furthermore, for each prey type separately, the likelihood of constriction increased with increased prey struggling, while relative prey size was held constant. The context-dependent nature of constriction in T. elegans supports the notion that, as these snakes began feeding substantially on rodents, constricting behaviour was selected specifically to subdue these vigorously struggling prey. Thus, in this early evolutionary stage, prey struggling appears to be both a proximate cue and a selective factor promoting the spread of constricting behaviour. In some snake species representing more ancient origins of constriction, the behaviour is elicited by pre-strike cues that appear to be correlated with struggling ability. Therefore, if the behaviour of T. elegans is taken as a proxy for an early evolutionary stage of constriction in general, we can infer an evolutionary shift in the stimulus control of the behaviour. Our results suggest a general conceptual model for the sequence of evolutionary changes in the stimulus control of behaviour in which the critical cue changes from the actual object/event for which a response is beneficial to some feature correlated with that object/event.